1. Field of the Invention
This invention relates generally to polymer coated organic polymeric films, and more particularly to a heat-sealable vinylidene chloride copolymer coating composition for organic polymeric films with good automatic packaging machine runnability properties.
2. Description of the Prior Art
Organic polymeric films such as cellophane are typically coated on one or both surfaces with a vinylidene chloride copolymer (saran) coating composition to provide the film with better moisture barrier properties. Such coated films are heat-sealable and are used to make packages on automatic packaging machinery which requires that the film must have good machine runnability properties, possess adequate slip, be free of blocking tendencies, and be readily heat-sealable so that packages can be properly formed, particularly on automatic packaging machinery of the type employing crimped sealing bars. In addition to the runnability performance, when the film is used for wrapping foodstuffs of relatively high moisture content, it is necessary that the film possess antifogging properties so that moisture from the packaged product does not deposit upon the internal surface of the film package in minute droplets as a fog. This tendency of moisture deposit in the form of a fog on the interior surface of the package reduces transparency and greatly interferes with the effective display of the particular packaged article.
In order to provide a heat-sealable vinylidene chloride copolymer coated film with good machine runnability properties that is nonfogging, the conventional practice in the industry is to apply a second coating containing a wetting agent on top of the vinylidene chloride copolymer coating. A conventional wetting agent used in the topcoating is sodium lauryl sulfate. This second coating step requires a second coating tower, adding to the cost of the system, and since the basic vinylidene chloride coating is normally applied using organic solvents and the topcoating is normally applied using an aqueous solution of the wetting agent, complications arise, particularly with regard to drying, solvent removal, and solvent recovery. A typical example of such a topcoating composition is disclosed in U.S. Pat. No. 3,394,024 in which the antifog and release agent composition is applied to the surface of the film after application of the base coating of saran by passing it through an aqueous solution of the composition, removing excess solution from the film by passing it through a set of squeeze rolls, and thereafter drying it. The topcoating composition in this case is an aqueous solution of a water-soluble metal salt, such as magnesium acetate or calcium acetate, in combination with sodium lauryl sulfate.
Another conventional method of improving the machine performance of vinylidene chloride copolymer coated cellophane is disclosed in U.S. Pat. No. 3,375,215, while additives are incorporated in the vinylidene chloride copolymer coating composition itself to improve the machine performance. The additives disclosed for this purpose are candelilla wax in combination with a stearate salt such as calcium or ammonium stearate. While such additives do improve the machine performance of the coated film, the wettability characteristics of the coating are, however, impaired making uniform printing of the coated cellophane difficult. Moreover, film produced in this manner fogs and requires a topcoating step to produce a nonfogging surface.
It is therefore an object of this invention to provide a vinylidene chloride copolymer coating composition for organic polymeric films which provides, in one coating, improved runnability performance of the polymer coated film on automatic packaging machinery with the flexibility of imparting full or partial nonfogging characteristics without impairing the machine performance or heat-sealability of the coated film and that does not require a second or topcoating application of a release agent.